As Stevan has pointed out, the OA community would not be what it is today
without ArXiv, just as the internet today would not be what it is without
physicists wanting to communicate and Berners-Lee thus implementing a truly
distributed hypertext system (it is important to remember that the idea, and
even other implementations of hypertext systems, pre-date the WWW-protocol(s)
by decade(s)). I believe Stevan when he makes the case that dsitributed IRs
are the best way forward and that this will emerge from the efficiencies they
offer. We're still trying to get the snowball to start rolling down the hill,
but we do appear to be right at the peak now.

However, on the question of ArXiv, I believe that it serves more purposes
than simply OA and in getting there (near-100% OA) from here (less than 5% OA
globally) it is important that we consider these other uses. There are two
reasons for this. One is that the other thing that ArXiv does is valuable in
itself. The other is that in order to encourage moves to 100% OA, losing
something else in the process will cause resistance from some quarters.

The other thing that ArXiv does,a s well as performing as a CR for
peer-reviewed work, is to provide a swift feedback route for work in
progress. Academic work is a means of communication. The posting of
pre-prints on ArXiv spontaneously generates significant feedback for many of
the posters. The reasons it is useful are multiple but IMHO the two main ones
are speed and the imperfection of peer review.

Speed: presently it can take years for a major peer-reviewed article to
appear. Some of this is due to necessary quality-control. Some of it is due
to the inefficiencies of the current system, still grounded in mid-20th
century technologies in many publishers. Some of it is due to the print
publishing method, which requires gathering a certain number of articles
together into an issue and so all of them are delayed until the final one is
ready to appear. In fasdt-moving fields such as physics and computer science,
this delay is too long and seriously reduces the utility of the articles when
they apepar. Computer Science has its own long-established solution to this,
in the fully-refereed conference proceedings, produced for publication at the
time of the conference. This is a tradition that almost no other field has.
New online journals such as the LMS JCM (http://www.lms.ac.uk/jcm) are
another attempt to keep this shrot (that one is also a gold OA journal, but
that's neither here nor there really).

The imperfection of peer review: sometimes very god work is not that well
presented, particularly by researchers in the early stages of their career.
Their work can be denied publication or severely delayed any distribution, by
the peer review process. In other cases, there are entrenched viewpoints
within fields and unorthodox work can find it difficult to find a publication
route in a journal of sufficent status.

So, what we have with ArXiv is a palce where, in a parallel strand to the
formal peer-review process, readers are willing to take the time to quickly
skim through pre-print work, the list focussed in some way onto a
sufficiently small subset of the total papers deposited each day/week/month,
and read some of them in detail. Sometimes this is done purely as a way of
keeping up with the zeitgeist in a particular field, sometimes it is done as
a way of finding potential collaborators in emerging new areas of research,
sometimes it is done as a means of finding the best new researchers either to
collaborate with or to try and attract to one's own team. There are many
reasons that researchers do this, and they do do it. There are a number of
stories (a number of them validated) of highly senior researchers, including
some nobel laureates even, who have been engaged enough in the community of
ArXiv, to send comments, encouragement and proposals of collaboration, to new
(or even not-so-new) researchers whose work they have seen on ArXiv and been
impressed by.

To conclude. While we push our way towards universal OA via the IR route, it
is in our best interests (both philosophically and pragmatically) to ensure
that we find ways to retain the benefits of the communities such as the
ArXiv, rather than ignoring these separate benefits.